Is it possible to change the dashmode sound?

Yes.
it can be changed using SRB2 Lua scripts!

SRB2 Lua has functions to stop sounds (such as S_StopSoundByID).
You can apply this to a character using hooks
(I would use PlayerThink or ThinkFrame, depending on which works as expected, since they behave differently) and stop the sound.
You can also remove the flashes, though that requires creating a mobj and applying practically all of the player's mobj properties to it.
 
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